Found by JasonB and I can reproduce on all `community-` servers so it doesn't seem like a recent regression.
user A: reacts with thumbs up emoji
user B: opens emoji picker to react with thumbs up emoji, and selects it in the picker
emoji reaction count does not increase from 1 to 2. However, hovering over the emoji reaction includes both user A and user B
Emoji reaction count increases from 1 to 2 and hovering over the emoji reaction includes both users A & B
See attached video.